1. Field of the Invention
The present invention relates to an input protection circuit for a signal processing circuit, such as a microprocessor or the like.
2. Description of the Prior Art
In an automotive vehicle or the like, a system is used which applies an output signal from each of temperature, pressure and like sensors to a signal processing circuit, such as a microprocessor or the like, and controls respective parts, such as an engine or the like, in accordance with the sensor output signal. There are cases where a power source voltage, e.g. of a battery, is applied as a power source voltage V.sub.CC to the signal processing circuit after being converted by a stabilized power source circuit into 5 V, for instance, and the power source voltage V.sub.B as of the battery is provided directly to the sensor. In such a system, the power source voltage V.sub.B is immediately applied to the sensor when turning ON a power source switch, but since the output voltage V.sub.CC of the stabilized power source circuit does not immediately rise up, the level of the sensor output signal may sometimes exceed the voltage applied to the signal processing circuit.
In general, since the signal processing circuit, such as a microprocessor, is broken down when the input signal level becomes higher than the power source voltage applied thereto, it is necessary that the input signal be provided to the signal processing circuit after the output voltage from the stabilized power source circuit has risen to its highest level. To meet this requirement, it is considered to provide a switch which applies the power source voltage to the sensors after complete rise-up of the stabilized power source circuit, but an arrangement including control of the switch becomes complex because the sensors are disposed at various positions.